1. Field of the Invention
This invention relates to artificial cardiac pacemakers, either external or implantable having an atrial pulse generator, the stimulating frequency of which is controlled by the sensing of ventricular events.
2. Description of the Prior Art
The implantable cardiac pacemaker, shown in U.S. Pat. No. 3,057,356 and subsequent patents permits innocuous, painless, long-term cardiac stimulation at low power levels by utilizing a small completely implanted transistorized and battery operated pulse generator connected via a flexible lead bearing an electrode directly in contact with cardiac tissue. Most pulse generators consist of a stimulating circuit and a sensing circuit both of which draw current from the battery. In the presence of complete heart block, an asynchronous pulse generator with only a stimulating circuit may be used, however, in most instances, noncompetitive triggered or inhibited pulse generators having the sensing circuit are preferred and dominate the pacemaker market. The demand, synchronous or triggered pulse generators are especially useful in patients with spontaneous cardiac activity because of their ability to sense intrinsic cardiac rhythm (atrial or ventricular depending on variety and electrode position), and to alter the pacemaker output accordingly. Such pacemakers are shown for example, in U.S. Pat. Nos. 3,253,596 (P-wave synchronous), 3,478,746 (ventricular inhibited) and are described in the pacing literature.
More recently, attention has been paid to the physiological aspects of cardiac pacing therapy and particularly to pacing systems for maintaining synchronous atrial and ventricular depolarization of the heart. In early atrial synchronized (or A-V synchronous) pacing, atrial depolarization is sensed through one electrode, and after an appropriate delay the ventricle is paced through a different electrode, thereby restoring the normal sequence of atrial and ventricular contraction and allowing the pacer to respond to physiologic needs by increasing its rate. Below a predetermined minimal atrial rate, however, the pacemaker reverts to its basic ventricular pacing rate. In atrial synchronous ventricular inhibited pacers of the type described in U.S. Pat. Nos. 4,059,116 and 3,648,707, the ventricular depolarizations are also sensed and inhibit or reset the timing of the ventricular stimulating pulse generator.
A more complex method of restoring synchrony is by the atrial ventricular sequential pacing of the type described in U.S. Pat. Nos. 3,595,242 and subsequent patents which possess atrial and ventricular pulse generators and associated electrodes and a ventricular sensing circuit. In atrial ventricular sequential pacing, the atria and ventricles are paced in proper sequence, the atrial and ventricular pulse generator timing circuits being reset on sensing spontaneous ventricular activity.
Finally, copending U.S. patent application Ser. No. 940,694 filed Sept. 8, 1978, now abandoned for continuation application, Ser. No. 120,237 filed Feb. 11, 1980, and assigned to the assignee of the present invention, discloses a pacemaker which, if required, may stimulate the atrium and/or ventricle on demand and which is able to maintain synchrony as the sensed atrial rate increases. A pacemaker of this type is capable of distinguishing between bradycardia and normal heart function and to provide atrial and/or ventricular pacing in the following modes: inhibited in the case where the atrium and ventricle beat at a sufficient rate; atrial demand in instances where the atrium is beating at an insufficient rate and must be stimulated whereas the ventricle properly follows; atrial synchronous when the atrium depolarizes at a sufficient rate but the ventricle does not follow within a prescribed A-V interval; and dual demand when neither the atrium and the ventricle spontaneously depolarize at the desired rate.
In all of the pacemakers of the types described above where an atrial stimulation is provided, the atrial pulse generator posseses a timing circuit having a predetermined escape interval. The timing circuit may be reset in certain instances by sensing of a ventricular and/or atrial depolarization detected prior to the timing out of the escape interval. In recent versions, the atrial (and ventricular) escape interval may be remotely programmed to provide a number of basic pacing rates extending batween a MINIMUM and MAXIMUM possible rate. Thus, if the natural atrial rate exceeds the basic interval of the atrial pulse generator timing circuit, the atrial pulse generator will be inhibited. Similarly, if the sensed ventricular depolarizations occur at a rate exceeding the basic rate of the atrial pulse generator timing circuit, then it again will be inhibited. In the former case, no physiological harm is done, since the heart is beating at a rate that is within the MAXIMUM and MINIMUM desired rate. In the latter case, however, if the ventricle of the heart is beating at a rate exceeding the atrial and ventricular escape intervals, the atrium is not depolarizing in synchrony with the ventricle and the hemodynamically desirable synchronism is lost. The patient may then suffer a loss of cardiac output at a time when the patient's physiology is demanding an increased cardiac output. That is, when the ventricle is contracting at a rate exceeding the atrium, it indicates that the patient is undergoing some stress or exercise that requires a greater cardiac output.
For example, if a patient has an absolute sinus bradycardia (that is, is devoid of an underlying atrial heart rhythm), then if an increasing load is applied to the patient's heart, the heart rate of the ventricle may increase to a rate exceeding the preset rate of the atrial pulse generator timing thereby inhibiting atrial stimulation. The atrium, no longer pumps in synchrony with the ventricle and the patient loses the atrial contribution to cardiac output. Thus, in a time of need, the patient may suffer cardiac insufficiency.
In other patients the sinus rhythm may become irregular as the load on the heart is increased through exercise, and the atrial ventricular synchrony again becomes disturbed. In such cases, it would be desirable to maintain the atrial ventricular synchrony at rates in excess of the preset atrial and ventricular rates of the pacemaker.
Similarly, it would be desirable to reestablish atrial and ventricular synchrony in instances where ventricular extrasystoles manifest themselves at irregular intervals. In the prior art pacemakers the extrasystole is sensed and inhibits both the atrial and ventricular pulse generators until a newly established escape interval times out. In such instances, the atrial and ventricular synchronism is lost and the heartbeat is erratic. It would be desirable to pace the heart in such a manner as to restore normal cardiac output as quickly as possible on the occurence of an extrasystole.
In addition it would be desirable to recognize and treat tachycardia. In conventional tachycardia detection pacemakers, tachycardia is detected by sampling the ventricular rate, and be it a physiologic or a pathologic tachycardia, the fixed limit of e.g. 150 BPM provokes interruption measures of the tachycardia breaking pacemaker. However, in a given patient, the high heart rate does not necessarily indicate whether a pathologic tachycardia is present; it might be as well a normal frequency as a response to exercise. Hence, conventional tachycardia breaking pacemakers would try to break this fast sinus rhythm, and this clearly would be dangerous, as a normal sinus tachycardia cannot be broken, but a lot of extra cardiac activity would be produced. In the same patient, a frequency of 120 could mean a pathologic (reentry-) tachycardia lending itself to interruption, whereas a frequency of 150 BPM in the same patient could be a physiologic tachycardia. This means, that the actual frequency of the heart is a bad indicator of tachycardia to be interrupted.
Conventional treatment calls for sensing such arrythmia rates in the ventricle and pacing the ventricle at higher (overdrive), lower (underdrive) or variable pacing rates for a period of time.
Such pacemakers may not be able to distinguish a single PVC from the first tachyarrythmia beat and may go into the tachycardia treatment mode unnecessarily.